1. Field of the Invention
The present invention relates to a C-MOS oscillator circuit and, more particularly, to an oscillator circuit intended to be used in quartz wrist watches.
2. Description of the Prior Art
The present trend in this field is to seek to lower the power consumption of integrated electronic circuits as much as possible, so as to achieve independent operation for several years for these watches which can contain only batteries of a small size and very low energy capacity. Equally important, for economic reasons, is the desire to design circuits which are as insensitive as possible to the inevitable variations encountered in manufacturing processes.
An important parameter for any MOS circuit is the value of the threshold voltage of the transistors. Although the differences between the threshold voltages of the transistors of one and the same circuit are normally extremely small (of the order of one-hundredth of a volt), the same is not true of the differences between the threshold voltages of the transistors from one circuit to another. These differences can, in fact, reach several tenths of a volt. So as to incur little waste during manufacture, it is very important that the functioning of the circuits is not prejudiced by such variations.
Most of the electronic watches on the market at present have a quartz crystal of 32,768 Hz as the time base. The frequency divider and the system needed to drive a stepping motor or to control a digital display consists of C-MOS circuits. This technique is highly suitable for all those circuits where the transistors in effect represent "on-off" elements. The same has not been true, however, of the oscillator circuit which is an analog circuit. It would be highly desirable, from an economic point of view, to be able to integrate the oscillator circuit by means of those very same processes required for all the other circuits of the watch.
To produce quartz oscillators having a very low power consumption, resistors have been used, particularly diodes made of polycrystalline silicon, which can have relatively high resistances. Layers of silicon are deposited by means of CVD processes on a layer of silicon dioxide which are subsequently doped "p" and "n", so as to form diodes, the characteristics of which approximate, more or less, those of a resistor. These characteristics are difficult to reproduce and, consequently, the need to resort to resistors represents a considerable disadvantage.
A C-MOS oscillator circuit which finds very widespread use at the present time is that which is the subject of French Pat. No. 2,110,109. The active element of the circuit consists of an inverter supplied by a source of d.c. voltage. A gate-bias resistor of a sufficiently large value (above 10 M.OMEGA.) to limit the losses is connected between the output and input of the inverter, in parallel with the quartz resonator. Two capacitors are connected between a terminal on the voltage source and the input and output of the inverter, respectively. One of these capacitors is variable so as to enable the oscillation frequency to be adjusted.
Apart from the fact that it contains a resistor, this oscillator circuit has the following disadvantages: since the transconductance of the transistors increases greatly with the amplitude of their control voltages, the amplitude is limited by overexciting the transistors. The result of this is high power consumption. Furthermore, as is well known, overexciting increases the phase shift between the output and input voltages of the active transistors, thus reducing the frequency stability of the oscillator. Furthermore, to ensure that oscillation starts, it is necessary for the two transistors to be conductive at the same time, which requires, for this circuit, a supply voltage greater than the sum of the threshold voltages of the two transistors. This creates severe demands on the tolerances permitted for the threshold voltages of the transistors when the supply voltage is low, as is the case in an electronic watch.
In most electronic watches presently available, the threshold voltage of the transistors used in digital circuits can be between approximately 0.3 volts and V.sub.p -.DELTA.V, where V.sub.p represents the voltage of the battery and .DELTA.V the minimum difference between the control voltage and the threshold voltage of the transistors of the frequency-divider circuits. If the threshold voltage were below 0.3 volts, the current resulting from low-inversion operation of the transistors of all circuits could cause an unacceptable consumption of energy. For the oscillator of the French patent, the sum of the threshold voltages of the two types of transistors must, therefore, be between 0.6 volts and V.sub.p -2.DELTA.V. For example, for a supply voltage of 1.2 volts, and assuming that a difference .DELTA.V of 0.1 volt is sufficient to ensure that oscillation starts, the sum of the threshold voltages of the P-type and N-type transistors can only be between 0.6 and 1 volt, implying much stricter tolerances for the manufacture of the oscillator than for that of the digital circuits. For the latter, even allowing that .DELTA.V is 0.2 volts, the threshold voltages of each type of transistor can be between 0.3 and 1 volt.
One solution which makes it possible, to a certain extent, to avoid overexciting the transistors and consequently to reduce the current consumption is the subject of Swiss Pat. No. 596,598. The oscillator circuit described in this patent is illustrated in FIG. 1 of the present drawings. However, this oscillator circuit retains the other two disadvantages of the circuit of the French patent, that is, it contains a resistor R and the demands regarding the tolerances for the threshold voltages of the transistors are just as severe. In fact, during starting, capacitor C.sub.p is discharged and the gates of transistors T.sub.1 and T.sub.2 are at the same potential. Since these transistors must, under these conditions, conduct simultaneously, the supply voltage must be greater than the sum of their threshold voltages. Another disadvantage results from the fact that the current passing through the transistors T.sub.3 and T.sub.4 depends, in the equilibrium state of the oscillation, on the values of the threshold voltages, i.e. the control conditions also depend, in this case, to a certain extent on the threshold voltages.
The Swiss Patent application published under No. 613,357G describes an oscillator, illustrated in FIG. 2 of the present drawings, in which two complementary transistors are biased separately for the purpose of reducing energy dissipation. In this oscillator, the gates of the P-type and N-type transistors T.sub.1 and T.sub.2 are connected, respectively, to the negative terminal and to the positive terminal of the supply source by means of resistors R.sub.1 and R.sub.2, respectively. Two coupling capacitors C and C' connect the gates of T.sub.1 and T.sub.2, respectively, to one terminal of a resonator Q, the other terminal of which is connected to the output of the inverter.
Such a solution makes it possible, in principle, to reduce the supply voltage to a value a little above the highest threshold voltage of the two transistors, but does not eliminate the problem of overexciting the transistors and the current consumption remains relatively high. Moreover, this oscillator contains resistors.